Modern electronic semiconductor components typically require relatively low DC voltages to operate. Of course, residential power lines provide relatively high voltage alternating current. Commonly, the alternating current provided by conventional power lines is connected to a transformer in order to step down (or up) the voltage to a desired level. The transformer output is then rectified, filtered and regulated to produce the direct current voltages required by solid-state electronic equipment.
As performance requirements for electronic circuitry have become more stringent and the operating tolerances of certain high-precision electronic components have narrowed, it has become critical to provide highly accurate voltage regulation of rectified and filtered power supply voltages. Voltage fluctuations in the voltages produced by unregulated DC power supplies can be attributed to a number of sources, including power line voltage variations, heating of power supply components, variations in load current, etc. Because nearly all semiconductor devices operate best at a constant power supply voltage (some devices will be destroyed upon the occurrence of even small power supply over-voltages and will fail to operate or operate unreliably upon the occurrence of small undervoltages), and will produce far more accurate outputs when operated under constant supply voltages, nearly all modern semiconductor power supplies provide some sort of voltage regulation.
Many techniques exist for voltage regulating a direct current voltage. For instance, a simple voltage regulator may comprise nothing more than a Zener diode and a series-connected resistor. Monolithic voltage regulators are presently commercially available which provide highly accurate voltage regulation at relatively low cost. When used in conjunction with pass transistors, these monolithic voltage regulators can regulate very high current DC voltage sources from output voltage fluctuations. Unfortunately, many effective voltage regulating schemes achieve voltage regulation at the expense of efficiency. Series-type voltage regulators often produce relatively high power losses in achieving good voltage regulation. Moreover, such voltage regulators typically provide voltage regulation for over only a narrow range of output voltage set points.
Switching regulators are used when it is necessary or desirable to minimize power losses which would otherwise occur in series voltage regulators or when large variations in input voltages or output currents may be likely to occur. Switch-mode power supplies are thus generally regarded as the most efficient of regulated power supplies because they are capable of achieving excellent output voltage regulation with little wasted power over a wide range of input voltages and output currents. Switch-mode power supplies typically comprise an unregulated, relatively high-voltage DC power supply, one or more switching elements, an output transformer, output rectifying and filtering components, and a control circuit. The unregulated DC voltage is selectively applied to the output transformer via the switching element or elements. The output of the output transformer is rectified and filtered to produce a DC output voltage. The output voltage level is measured and used to pulse-width modulate the control signal, which controls the switching duty cycle of the switching elements. The output voltage may be increased by turning the switching elements on for a greater proportion of the total time, and may be decreased (down to zero, if necessary) by decreasing the duty cycle of the switching elements. Using this arrangement, little power is dissipated in heating the components responsible for regulating the voltage.
Efficiency of the supply depends mainly upon the switching and saturation losses of the switching elements. A number of techniques have been proposed for increasing the switching speeds of switch-mode power supply switching elements, thereby increasing the overall efficiency of the supply. See, for example, the following prior-issued U.S. Pat. Nos.:
4,342,956--Archer--Aug. 3, 1982 PA0 4,322,785--Walker--Mar. 30, 1982 PA0 4,308,577--Mentler--Dec. 29, 1981 PA0 4,060,758--Wright--Nov. 29, 1977 PA0 3,986,052--Hunter--Oct. 12, 1976 PA0 3,983,418--Wallace--Sept. 28, 1976 PA0 3,975,672--Lachocki--Aug. 17, 1976 PA0 3,838,329--Michelet et al--Sept. 24, 1974 PA0 3,641,424--Kuykendall--Feb. 8, 1972 PA0 3,600,666--Gliever--Aug. 17, 1971 PA0 3,579,090--Madsen--May 18, 1971 PA0 3,559,040--Fickenscher--Jan. 26, 1971 PA0 3,378,758--Goodenow--Apr. 16, 1968 PA0 3,304,489--Brolin et al--Feb. 14, 1967 PA0 3,361,955--Typke--Jan. 2, 1968 PA0 U.S. Pat. No. 4,092,708 to Gerding et al (issued May 30, 1978); and PA0 U.S. Pat. No. 4,288,831 to Dolickian (issued Sept. 8, 1981). PA0 U.S. Pat. No. 4,233,557 to Alberkrack (issued Nov. 11, 1980); PA0 U.S. Pat. No. 4,378,585 to Bete (issued Mar. 29, 1983); PA0 U.S. Pat. No. 4,228,493 to de Sartre et al (issued Oct. 14, 1980); PA0 U.S. Pat. No. 4,209,826 to Priegnitz (issued June 24, 1980); PA0 U.S. Pat. No. 4,129,791 to Sato (issued Dec. 12, 1978); and PA0 U.S. Pat. No. 4,001,663 to Bray (issued Jan. 4, 1977).
As is well known, switching transistors are not capable of instantaneously switching on upon the application of an input signal due to "storage time" effects. When a bipolar junction transistor is off, its base charge is nominally zero. When it is turned on, some finite time must elapse to allow charged distributions within the various layers to become established. Negative charges are provided by electrons crossing the emitter junction, while positive charges are furnished by base current. If the transistor is driven into saturation (which is normally desired in switching applications in order to turn it fully on and therefore reduce the voltage drop across it), an excess minority-carrier charge is accumulated comprising a uniform saturation charge (which makes no contribution to the current flow) and another charge which creates the density gradient necessary to sustain current flow by diffusion across the base. Thus, when the transistor is driven on, its collector current will not attain its final value until both the excess accumulated charge and the charge which establishes the density gradient have been established. On the other hand, a transistor which is on and in saturation cannot be turned off until both stored charges have been removed. During the interval when the saturation charge is being removed, the transistor collector current will not diminish; collector current will only begin to fall off as the charge establishing the density gradient begins to dissipate. See Taub, H. and Schilling, D., Digital Integrated Electronics, 44-49 (McGraw-Hill 1977).
Hence, a high-speed model of a bipolar junction transistor contains a capacitance shunted across its base which limits the switching speed of the device. In addition, lead wire connections (which connect the device leads to the transistor structure layers) have associated with them small amounts of resistance. The connection resistance of the transistor base lead to the base layer limits the amount of current which can instantaneously flow in or out of the base lead of the device, further limiting the device switching speed (since it limits the speed which the storage charge can be applied or removed). The delays in switching of the transistor caused by the combined effects of saturation charge and lead wire resistance (i.e. the delay between the time when base current is applied to the transistor to the time when the collector current reaches maximum value, and the time delay between the time when the base current begins to be removed from the base of the transistor to the time when collector current is reduced to zero) is called "storage time."
The effects of time delays associated with switching transistor storage time are especially critical in switch-mode power supplies utilizing a pair of switching transistors connected in series across a center-tapped unregulated DC source. Such power supplies operate by alternately connecting the primary winding of the output transformer between the center tap and the positive lead of the DC source and between the center tap and the negative lead. Minimum voltage output is, of course, achieved by reducing the duty cycle (ratio of transistor on time to off time) of the switching elements to approximately zero. Maximum voltage output is achieved by increasing the duty cycle of each of the switching elements to approximately 50%. It will be understood that if the duty cycles of the switching elements exceed 50%, the switching elements will be placed in series directly across the full output of the DC source, causing them to self-destruct due to overcurrent. It is therefore essential that both switching elements not be turned on simultaneously.
Due to storage time of the transistors, accurate and predictable control of the on and off times of each transistor is generally impossible to achieve. For this reason, prior art power supplies are designed with "dead time," i.e. a period is provided during each half cycle wherein neither transistor is on. See, for example, U.S. Pat. Nos. 4,342,076 to Rosswurm et al (issued July 27, 1982); 4,150,424 to Nuechterlein (issued Apr. 17, 1979); and 4,087,850 to Koizumi (issued May 2, 1978). Unfortunately, dead time limits the maximum duty cycle of each switching transistor to well below 50%. Such schemes must usually further limit maximum duty cycle in order to compensate for variations in the switching speeds of different transistors. Therefore, such prior art power supplies undesirably limit the range of voltage regulation capable of being provided. An alternative solution is to use switching transistors with matched switching characteristics. However, this solution is expensive (since it requires more expensive components) and makes component replacement a difficult task. Other solutions have been proposed; see, for example, U.S. Pat. No. 4,308,576 to Clark, Jr. (issued Dec. 29, 1981), which teaches using the interaction between two halves of center-tapped primary windings of the power transformer of a switch-mode power supply and series-connected switching transistors to force the transistors into nonconductance when an overcurrent occurs.
Yet another disadvantage associated with prior art switch-mode power supplies is the requirement of a separate internal power supply used to power the control circuitry. Active devices are used for sampling the supply output voltage and for producing the control signals necessary to switch the switching elements on and off. These active control devices, of course, require power of their own to operate. Many prior art switch-mode power supplies provide a separate DC power supply solely for powering the control circuitry. See, for example, U.S. Pat. No. 4,160,945 to Schorr (issued July 10, 1979). See also U.S. Pat. No. 4,030,024 to Chambers et al (issued June 14, 1977), which discloses a preregulator power supply for the control and drive circuits of the power supply which permits direct coupling to the power switching regulator device without requiring isolating devices. U.S. Pat. No. 3,702,434 to Ryan (issued Nov. 7, 1972) proposes to utilize a voltage-dropping resistor and a capacitor to reduce the voltage of the unregulated DC source of a switch-mode power supply (which typically produces a relatively high voltage, perhaps several hundred volts) to a voltage level suitable for powering the active control circuitry of the supply. This solution is relatively inefficient, however, because the unregulated source voltage level must be reduced substantially, requiring the dissipation of substantial amounts of power in the voltage dropping resistor as heat. The resulting power supply is larger in physical size, runs hotter and is more inefficient.
A better way of providing power to the control circuitry of a switch-mode power supply is to use the regulated output of the supply itself to supply voltages to the control circuitry. This is a deceptively simple solution, however, as it will be realized upon reflection that the control circuitry of a switch-mode power supply must operate to initially turn on the switching transistors before the supply can provide any output. Undaunted by this complication, prior art designers have proposed various ways to initiate switching of the switching transistors until the supply begins to generate an output voltage (which is then used to power the control circuits). See, for example, U.S. Pat. No. 4,323,961 to Josephson (issued Apr. 6, 1982) (primary circuit bleeder resistor voltages initiate switching of switching transistors; a second secondary winding of the output power transformer provides power to control circuitry thereafter); U.S. Pat. Nos. 4,246,634 to Purol (issued Jan. 20, 1981); 3,660,750 to Businelli (issued May 2, 1972); and 4,164,014 to Crowe et al (issued Aug. 7, 1979).
A very desirable feature in a switch-mode power supply is the so-called "burp mode" of operation, which is essentially a current limiting shut-down feature with automatic restart. When excessive current is drawn from the output of the power supply, a current limiting circuit senses the excessive current level and cuts back the output voltage of the supply (typically to nearly zero volts), thereby effectively shutting down the power supply in order to prevent damage to the power supply components (such as the switching transistors) from overcurrent. Some power supplies require a manual reset by a user once overcurrent shut-down has occurred (see, for example, U.S. Pat. No. 3,377,540 to Meyer issued Apr. 9, 1968); while this is desirable for some applications, it is generally not suitable for applications where an operator does not constantly monitor the operation of the device powered by the supply. For these more automatic applications, an automatic restart feature has been developed wherein the power supply shuts itself down upon the occurrence of an overcurrent, and subsequently, after a time, restarts itself automatically. If the overcurrent condition still exists, the power supply once again shuts down, and subsequently restarts. This process may continue indefinitey without damage to any of the power supply components. "Burp mode" is descriptive terminology for this type of current limiting feature. The following prior-issued U.S. patents disclose power supplies with a "burp mode" feature:
Unfortunately, relatively complex circuitry is normally required to achieve this very desirable power supply behavior.
Another problem ordinarily associated with switch-mode power supplies is providing sufficient isolation between the power supply output and power supply primary circuits. Many safety regulations and safety standards require extremely high DC isolation between the output terminals of a power supply and power supply internal high voltage potentials. Power supplies must be designed to physically isolate secondary circuits from primary circuits in order to ensure that high voltage potentials can never reach the power supply output to harm users or damage load components. In order to achieve high DC isolation between power supply primary and secondary circuitry, the prior art has made extensive use of conventional opto-coupler coupling devices. An opto-coupler is a light emitting diode optically coupled to a light-sensitive transistor such that the transistor conductance is dependent upon the LED output luminance. No electrical or physical connection between the input and the output of the opto-coupler is necessary, so that extremely high input-to-output DC isolation is obtained. The following references disclose the use of an opto-coupler to achieve DC isolation between the output and primary circuits of a power supply:
In the prior art, the analog voltage level of the output of the power supply is monitored to control switch-mode voltage regulators. In the prior art, opto-couplers are typically connected in an analog mode in such applications (i.e. the power supply output voltage varies the intensity of the LED, which in turn varies the conductance of the light-sensitive transistor through a range). Unfortunately, it is generally necessary to provide compensation in order to account for variations in the gain of the particular opto-coupler unit used, since the gain of such devices can vary substantially from one unit to another. Unless some way of "tweaking" or otherwise compensating each power supply control circuit for the particular gain of the opto-coupler is provided, inaccurate regulation or poor or unstable closed loop response may result.
Many complex and sophisticated power supply functions can be provided by a switch-mode power supply controlled by a digital controller. U.S. Pat. No. 4,352,153 to Voyer (issued Sept. 28, 1982) discloses a power supply including a digital controller which automatically corrects for imbalance in the operation of the switching transistors. Unfortunately, the expense of digital controllers such as microprocessors is relatively high, and justifiable only for extremely critical applications. Generally, it is more cost-effective to use discrete components (in combination with monolithic power supply chips, which are common, widely available and relatively inexpensive) to achieve sophisticated power supply control features whenever possible.